Conventional gear-driven single rotation turbo props are typically used at lower cruise speeds where they provide good performance and high efficiency. At higher cruise speeds, it is typically required to use a ducted turbo fan to produce the relatively high thrust required. A scaled up version of a conventional turbo prop engine suitable for powering an intermediate, or large sized transport aircraft at Mach 0.7 and 0.85 cruise speeds and high altitude would require excessively larger propeller diameters than is conventionally possible. The limiting factors of the single rotation propeller is the low power disk loading and reduced efficiency at high subsonic flight speeds.
The turbo machinery for aircraft having propellers are generally arranged to use a speed change gearbox in order to reduce the speed of the propeller rotor relative to the speed of the turbine. The speed change gearbox provides the method for a more optimum propeller tip speed for high efficiency along with a high speed, smaller diameter turbine drive shaft and a high speed turbine with fewer stages. However, a gearbox and associated accessories result in a significant increase in engine complexity, weight and inefficiency.
Unducted, counterrotating aft engines have been developed such as the GE 36 engine frequently referred to as the UDF engine. Such engines are direct driven without gearboxes. They are, however, generally mounted on the aft portion of the aircraft fuselage. Mounting such an aft fan engine on the wing of an aircraft is difficult due to the engine support pylon being forward of the large diameter fan.
A ducted, counterrotating gearless front fan engine forming a high bypass ratio engine has been submitted by the present inventor and assigned to the present assignee and is now co-pending under application Ser. No. 902,341, filed on Aug. 29, 1986.